Electrostatic atomizer and method of producing atomized fluid sprays

ABSTRACT

Fluids are atomized using a miniaturized electrostatic microinjector. The microinjectors are capable of producing uniform droplets in several spray modes, and metering and dispersing very small volume fluids. The atomizer is useful in carburetion systems for internal combustion engines, to prepare samples for analytical methods such as MALDI, for fluid filtration and separation, and in other applications.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of provisional application60/329,137, filed Oct. 12, 2001.

BACKGROUND OF THE INVENTION

[0002] This invention relates to an atomizer that creates liquiddroplets through application of an electrical field.

[0003] Many processes depend on the formation of liquid droplets ofcontrollable size. Examples of this include internal combustion engines,ink jet and bubble jet printers. Performance of most combustion enginesdepends strongly on how well the liquid fuel is injected into thecombustion chamber or inside the carburetion system. The process ofcombustion is limited by the size distribution of fuel droplets sprayedinto the air stream. The purpose of spray atomization is to create avery small size distribution of droplets with high surface area for heatand mass transfer. Typically, heat and mass transfer scale as d⁻² (d isthe droplet diameter) while the aerodynamic response time of thedroplets scales as d². Thus, the smaller the droplets, the more rapidlythey evaporate while they are given more time for evaporation within theair flow stream.

[0004] While light fuels like octane have low vapor pressure andevaporate fairly rapidly, heavy hydrocarbons such as diesel and JP8 willtake more heat and longer time to completely vaporize in the combustionchamber. It is therefore common to produce a much finer mist usinghigh-pressure atomizers in diesel engines. The injection pressuredelivered by plunger pumps to the spray nozzles in a diesel engineusually range from 1,500 to 7,000 psi. At these pressures, the dropletsrange in size from 10 to 100 μm with a Sauter mean diameter ofapproximately 50 μm.

[0005] In spark ignition engines, the issue of broad size distributionin the droplets causes less of a problem than in compression ignitionengines. In compression ignition engines, the fine droplets burn toofast, and the larger droplets don't follow the flow path, leading tounburned hydrocarbons emissions or the formation of deposits in theengine.

[0006] However, uniform droplet size distribution has been difficult toachieve using conventional high-pressure spray atomizers. The broadeningof the droplet size distribution can be attributed to several differentprocesses. The atomization process, which is characterized as highlychaotic at the onset of jet breakup, results in different break-upwavelengths and therefore different droplet diameters. Further, aftereach droplet is formed and is being issued into the flow stream, smallsatellite droplets form in its tail. These two mechanisms are inherentlyinterdependent in that the wavelength of the column of liquid injectedout of a nozzle (or what forms immediately after the liquid jet leaves anozzle) dictate the shape of the main droplets and number of trailingdroplets. Some of the droplets tend to coalesce after injection to formlarger droplets. The rate of collision and coalescence is a function ofthe turbulence intensity in the flow stream, the initial droplet sizedistribution and the number density of the droplets.

[0007] In some jet engines, fuel and air are mixed by dispersing thefuel into a high velocity stream of air, where the air turbulenceprovides the energy for atomization (so-called air-blast mixing). Thisapproach suffers from the drawbacks that (1) uniform droplets are notcreated and (2) atomization depends on the air velocity, which can vary.

[0008] Recent efforts have focused on the electrostatic dispersion ofthe fuel droplets to reduce coalescence. However, the potential payofffor focusing on this mechanism is extremely low. Researchers have beenvery interested in the process of jet instability and liquid columnbreakup. It turns out that the mode of growth of instability waves tendto lock in on external excitations, overriding the natural frequency ofthe fastest growing waves. For example, an acoustic force is commonlyused for controlling jet breakup and atomization. However, acousticscannot have direct impact on satellite droplet formation. A significantamount of research has also been vested in suppression of satellitedroplets, mainly in the ink jet and bubble jet printer industries. Theconcept of “tail cutting” has been explored and demonstrated inmicroinjectors using a recently developed thermal ink jet atomizer.Using diesel fuel, approximately 30 μm droplets have been issued out ofa 30 μm nozzle.

[0009] A method by which droplets of controllable size can be producedusing low energies and pressures would be desirable.

[0010] In other applications, it is desirable to be able to dispensesmall volumes of fluids in the form of small droplets. Conventionalmethods of atomizing fluids do not provide the fine control needed toatomize small quantities of fluids efficiently. This leads to poorresults and waste of the fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a side view, partially in section, of an embodiment of amicroinjector of the invention.

[0012]FIG. 2 is an isometric view of an embodiment of an atomizer havinga plurality of microinjectors of the invention.

SUMMARY OF THE INVENTION

[0013] In one aspect this invention is an atomizer for a liquidcomprising

[0014] A) at least one microinjector including (1) an orifice throughwhich the liquid is brought in contact with a pin emitter and (2) aconductive pin emitter extending outwardly from said orifice, the pinemitter having a radius of curvature in at least one location externalto said orifice of no greater than 500 μm;

[0015] B) means for introducing the liquid to be atomized through theorifice and to the pin emitter, and

[0016] C) means for connecting said pin emitter to a voltage source.

[0017] In a second aspect, this invention is a method of producingliquid droplets comprising

[0018] I) introducing a liquid into an atomizer comprising

[0019] A) at least one microinjector including (1) an orifice throughwhich the liquid is brought in contact with a pin emitter and (2) aconductive pin emitter extending outwardly from said orifice, the pinemitter having a radius of curvature in at least one location externalto said orifice of no greater than 500 μm;

[0020] B) means for introducing the liquid to be atomized through theorifice and to the pin emitter, and

[0021] C) means for connecting said pin emitter to a voltage source;

[0022] II) bringing the liquid into contact with the pin emitter, and

[0023] III) applying sufficient voltage to the pin emitter such that theliquid is emitted from the pin emitter as a plurality of droplets.

[0024] In a third aspect, this invention is a carburetion system for aninternal combustion engine, comprising

[0025] I) an outlet for a mixture of atomized fuel droplets and air;

[0026] II) an air inlet which is in fluid communication with said outletsuch that during operation air passes through said inlet, is mixed withfuel droplets and passes through the outlet;

[0027] III) an atomizer that is in fluid communication with said outletand which emits a plurality of fuel droplets into a stream of air thatpasses from the air inlet to the outlet, wherein said atomizer includes

[0028] A) at least one microinjector including (1) an orifice throughwhich the fuel is brought in contact with a pin emitter and (2) aconductive pin emitter extending outwardly from said orifice, the pinemitter having a radius of curvature in at least one location externalto said orifice of no greater than 500 μm;

[0029] B) means for introducing the fuel through the orifice and to thepin emitter, and

[0030] C) means for connecting said pin emitter to a voltage source.

DETAILED DESCRIPTION OF THE INVENTION

[0031] An illustrative embodiment of the atomizer of the invention isshown in FIG. 1. In FIG. 1, atomizer 1 includes microinjector 3.Microinjector 3 includes orifice 5 and pin emitter 4. Pin emitter 4 isexternal to orifice 3 in the sense that fluid to be atomized passesthrough orifice 5 to reach pin emitter 4, where it is atomized anddispersed as a plurality of fine droplets. Pin emitter 4 in thisembodiment is the terminus of hollow needle 9. Hollow needle 9 is inliquid communication with reservoir 7, which, in turn, is in liquidcommunication with conduit 8. The liquid to be atomized is introduced tothe orifice and pin emitter through conduit 8, reservoir 7 and the borein hollow needle 9. Microinjector 3 is supported by base support member2. In this embodiment, support member 2 is mounted onto base member 6.The internal walls of base member 6 and base 2 define reservoir 7.

[0032] The atomizer includes a means for connecting the pin emitter to avoltage source. In the embodiment shown in FIG. 1, needle 4, base 2 andbase support member 6 are all electrically conductive materials that arein turn connected or connectable to a voltage source, so that an appliedelectrical current applied to base support member 6 through line 10 isconducted through base 2 and needle 9 to pin emitter 5. Alternately,needle 9 may communicate directly with the voltage source via a wire,printed circuit or line that bypasses base 2 and base support member 6,or which passes through reservoir 7. Any type of circuitry that candeliver the required voltage and current to pin emitter 5 is suitable.

[0033] The embodiment shown in FIG. 1 is a preferred one, in which thepin emitter forms the tip of a hollow needle, and the fluid to beatomized is brought to the pin emitter through the needle bore. It isalso possible, but much less preferred, to design the microinjector suchthat the pin emitter protrudes through the orifice, so that the fluid tobe atomized passes through the orifice on the outside of the pinemitter, where it is dispersed into droplets. For example, a pin emitterof circular cross-section may protrude from a ring-shaped orifice thatis concentric with the pin emitter. In this less preferred embodiment,surface tension forces and/or an applied hydrodynamic pressure cause thefluid to pass through the orifice and wet the protruding surface of thepin emitter.

[0034] The atomizer may include a collector electrode, which is spacedat a distance from the pin emitter. The collector electrode is eithergrounded or in electrical connection with the voltage source, in whichcase the collector electrode is of the opposite polarity as the pinemitter. Any grounded part can function as the collector electrode.However, unless the distance between the collector electrode and pinemitter is small (in comparison with the radius of curvature of the pinemitter), the collector electrode has very little effect on dropletformation. However, the collector electrode may affect the trajectory ofthe droplets once they are formed.

[0035] During operation, voltage is applied to the pin emitter, creatinga charge on the pin emitter and a local electrical field with a gradientin the field strength. The electrical field induces pressure gradientson the fluid for driving its flow and atomization. These forces can beexpressed as $\begin{matrix}{f_{E} = {{Q\overset{\_}{E}} - {\frac{1}{2}E^{2}{\nabla{ɛ\left\lbrack {\frac{1}{2}\rho \quad {E^{2}\left( \frac{\partial ɛ}{\partial\rho} \right)}_{T}} \right\rbrack}}}}} & (1)\end{matrix}$

[0036] where ε is the dielectric permittivity of the fluid, ρ is themass density, Q is the electric field space charge density, T is thetemperature and E is the applied electric field strength. The first termon the right-hand side of Equation (1) represents the force on the freecharges present and gives rise to the so-called Coulomb force, which isthe primary driving force in most ion-drag pumps for pumping a liquid orgas in single-phase mode. The second and third terms are theelectrostrictive force and the dielectrophoretic (DEP) force. Q isdefined as $\begin{matrix}{Q = \frac{I}{\left( {u + {\mu \quad E}} \right)A}} & (2)\end{matrix}$

[0037] where I is the current, u is the bulk fluid velocity, μ is theion mobility, E is the electric field strength and A is the flowcross-section area.

[0038] The pressure rise produced by the electrical field is related tothe driving voltage and geometrical parameters. For a simple designinvolving laminar flow and a circular orifice cross-section, thepressure rise required to generate droplets is related to the dropletescape velocity, ion mobility and permittivity as follows:$\begin{matrix}{{\Delta \quad p} = \frac{ɛ\quad u^{2}}{6\quad \mu^{2}}} & (3)\end{matrix}$

[0039] where u is the average droplet escape velocity in m/s, μ is theion mobility in m²/volt-sec, and ε is the permittivity in C/volt-m.

[0040] The pressure gradient created by an electrical field can berelated to applied voltage according to the relationship:$\begin{matrix}{{\Delta \quad p_{e}} = {\frac{3}{2}{ɛ\left( \frac{V_{1} - V_{o}}{\delta} \right)}}} & (4)\end{matrix}$

[0041] where V₁ is the applied voltage, V₀ is the threshold breakdownvoltage (which is very small for liquids) and δ is the inverse of thesurface curvature of the pin emitter at the point of smallest radius ofcurvature (or, if smaller, the inter-electrode spacing).

[0042] Thus, at any given applied voltage, the electrical field gradientthat is created will be greatest at that point of the pin emitter atwhich the radius of curvature is smallest. Therefore, it is importantthat the radius of curvature of the pin emitter be small at one place atleast, so that the necessary applied voltages remain relatively small.For many fluids, electrical field gradients in the range of from about 1to about 1000 kV/mm, especially from about 5 to about 400 kV/mm,particularly from about 10 to 200 kV/mm are sufficient to initiate andcontinue droplet formation. Electrical field gradients of thesemagnitudes can be produced at applied voltages in the desirable range of100-25,000 volts, at microampere currents or less, when the radius ofcurvature of the pin emitter is no greater than 500 μm, preferably nogreater than 250 μm, even more preferably no greater than about 150microns, and especially from about 1- 50 μm. At these pin emitter sizes,pressure drops needed to obtain atomization are usually in the range ofabout 0.001 bar to 0.1 bar. These pressure drops are several orders ofmagnitude smaller than required in conventional types of atomizers.

[0043] The pin emitter may have, for example, a conical shape, acylindrical shape, a rectangular shape, a round tip, a sharp or pointedtip, or a more complex curvature. It is made of any material capable ofbeing charged in response to an applied voltage, with metals such assteel, aluminum, copper, silver, gold and platinum being of particularinterest. Pin emitters with sharpened tips are especially preferred. Theelectrical field gradient generated by the pin emitter is usuallygreatest at that location where the curvature of the pin emitter ishighest, and droplets preferentially form and are emitted at thislocation. In the case of a pin emitter with a sharpened tip, thesharpened tip is the region of greatest surface curvature, and dropletformation usually occurs there.

[0044] A particularly preferred type of microinjector is a hollow needlehaving a pointed or sharpened tip, having an outside diameter of up to 1mm, preferably up to 700 μm, especially from about 5 to about 400 μm,most preferably from about 10 to about 250 μm.

[0045] The microinjector is operated by applying a voltage to the pinemitter and bringing the fluid into contact with the pin emitter throughthe orifice. As discussed above, voltages required will depend somewhaton microinjector geometry and the particular fluid being atomized. Inaddition, the voltage required to initiate droplet formation variesdepending on whether the voltage is constant or pulsed. In general,however, applied constant voltages in the range of about 100 V to about25 kV, especially from about 1-20 kV, most preferably about 3-15 kV, aresuitable for producing fluid droplets.

[0046] For a given type of current and at a given mass flow rate,increasing voltage tends to reduce droplet size. This effect can beestimated using the relationship expressed by the Raleigh limit:$\begin{matrix}{I = {12\sqrt{2}\left( \frac{ɛ\quad \gamma_{o}}{d^{3}} \right)^{\frac{1}{2}}Q}} & (5)\end{matrix}$

[0047] where I is the current, ε is the dielectric constant of thefluid, γ is surface tension, d is the droplet diameter and Q is massflow rate through the orifice. The Raleigh limit gives the maximumcurrent flow for a given fluid at a given particle size.

[0048] Thus, the invention provides a way of making droplets ofpredetermined sizes (within some range) by varying the applied voltage.This effect will be dependent on the geometry of the system and thefluid (and waveform of the applied voltage), but is easily determinedempirically for any given system.

[0049] Applicants have also found that various spray modes can beproduced through varying the applied voltages, particularly when aconstant DC voltage is applied. At DC voltages near the thresholdvoltage for droplet production, the microinjector often operates in asingle droplet mode, in which individual droplets are produced atsignificant intervals. Increasing the voltage somewhat often creates alinear stream of droplets, due to their faster production. Increasingthe voltage more usually causes large numbers of more highly chargeddroplets to form. The electrostatic repulsion between these dropletswill cause them to form a dispersed cloud or mist having a spraydispersion angle that may range from about 20° to about 120° or more.This effect becomes greater with higher dielectric constant fluids.

[0050] Pulsing the applied voltage provide yet another method ofcontrolling droplet formation and allows higher mass flow rates to beachieved. Pulsing is used herein to refer to a variety of waveforms(such as, without limitation, square, sawtooth, sinusoidal, etc.) inwhich the voltage is variable with respect to time. The pulsed voltagemay be a simple alternating current. Pulsing frequency is advantageouslyin the range of from 10 to 5000 Hz, preferably 50-1000 Hz, especiallyabout 50-200 Hz. Pulsing the voltage tends to reduce the amount ofapplied voltage needed to initiate droplet formation, produce smallerdroplets at a given voltage, geometry and mass flow rate, and to favor aspray mode of operation. In addition, power requirements tend to begreatly reduced when a pulsed voltage source is used. Exemplary appliedvoltages (peak-to-peak) are from about 1 to 25 kV, especially from about3-10 kV, when the voltage is pulsed in the range of 50-200 Hz, althoughthis will depend somewhat on microinjector geometry, mass flow rates andfluid characteristics.

[0051] Currents per microinjector are typically in the range of 10 μA toabout 10 mA, especially from about 100 μA to about 1 mA, when a constantDC voltage is applied. However, current (and therefore power)requirements tend to be much smaller when a pulsed voltage is applied,at a given mass flow rate.

[0052] When the fluid is relatively non-polar, the droplets tend to behighly uniform. Although this invention is not limited to any theory,applicants believe that the electrical field generated by theminiaturized microinjectors helps to suppress the formation of smaller,satellite droplets, through generation of a dielectrophoretic (DEP)force. DEP force exists when the following two conditions aresimultaneously satisfied: (a) there is a gradient of the electric fieldstrength and (b) there is a change in the dielectric constant across theinterface separating the droplets and the air (or other fluid) intowhich the droplets are dispersed. The DEP force experienced by a dropletcan be expressed as: $\begin{matrix}{F_{e} = {\frac{\pi}{4}d^{3}ɛ_{0}{k_{1}\left\lbrack \frac{k_{2} - k_{1}}{k_{2} + {2k_{1}}} \right\rbrack}{\nabla{E}^{2}}}} & (6)\end{matrix}$

[0053] where d is the particle diameter, e_(o), is the dielectricconstant in vacuum, k₁ and k₂ are the relative dielectric constants ofthe liquid droplets and the surrounding fluid, and E is the electricfield strength. As the magnitude of the force depends on the term k₂-k₁,it is seen that the DEP force increases as the difference in thedielectric constants increases. Values of k₂-k₁ of at least 0.5 aredesirable, and values of at least about 0.8, especially of at least 1.0,are preferred. k is 1 for air and approximately 2 for non-polar fluidssuch as diesel fuel and most other heavy liquid fuels such as JP5 andkerosene. This difference in dielectric constant provides a significantchange in the dielectric constant giving rise to a measurable forceacting on the interfaces between the two fluids (i.e., the air andliquid fuel, in the case of injecting a fuel into air).

[0054] Conversely, more polar (higher dielectric constant) materialssuch as water often are dispersed with a broader particle sizedistribution.

[0055] Mixtures of materials are often dispersed in a bimodal ormultimodal pattern, even if those materials are miscible, if theirdielectric constants are significantly different. Under thosecircumstances, the component having the higher dielectric constant tendsto form a spray cloud with a relatively wide spray dispersion angle. Thelower dielectric constant component tends to form a spray cloud with amuch narrower spray dispersion angle. The resulting spray tends consistof a region, typically along the longitudinal axis of the spray cloud,which is rich in the lower dielectric constant material (because thedroplets are mainly droplets of the lower dielectric constant material,or because the droplets are enriched in the lower dielectric constantmaterial, or both), and another region, typically near the boundaries ofthe spray cloud, that is rich in the higher dielectric constant material(because the droplets are mainly droplets of the higher dielectricconstant material, or because the droplets are enriched in the higherdielectric constant material, or both.

[0056] This phenomenon provides the possibility of separating componentsof a mixture by isolating the portion of the spray that is rich in oneor the other material. The isolated material may be re-atomized one ormore times to improve the separation. This separation technique isuseful for isolating a component from a small volume of a mixture, evenif the materials are miscible, without using energy-intensive orexpensive techniques such as distillation.

[0057] The mass flow rate of the fluid to the microinjector is anothercontrol parameter. In many cases it is not necessary to supply the fluidto the microinjectors under any hydrodynamic pressure (i.e. fluidpressure other than that created by the application of voltage to thepin emitter) at all, so long as the fluid is brought into contact withthe pin emitter. However, if fluid is not constantly supplied to the pinemitter, droplet formation may become intermittent or droplet sizeinconsistent. A small applied hydrodynamic pressure can assure that aconstant supply of fluid reaches the pin emitter. It also tends toreduce the strength of the electrical field needed for dropletformation. Mass flow rate can affect droplet size, so controlling thisvariable through the control of hydrodynamic pressure offers anothermeans of controlling droplet size. On the other hand, if thehydrodynamic pressure is too high, mass flow rates exceed the rates atwhich droplets can form, or cause voltage requirements to increase,resulting in leakage, inconsistent performance or increased powerrequirements. Typically, an applied hydrodynamic pressure of about zeroto about 5, preferably from about 0.1 to about 2″ of water is sufficientto provide an acceptable mass flow rate of the fluid to themicroinjector. More or less viscous liquids may require more or lesshydrodynamic pressure to optimize mass flow rates and overall operation.Applied hydrodynamic pressure preferably is such that droplet formationand/or leakage of the fluid through the orifice will not occur unlessthe microinjector is operated through application of a voltage to thepin emitter.

[0058] Because only low (or no) applied hydrodynamic pressures areneeded for good operation, the atomizer does not require bulkyconstruction (to withstand high pressures) or large or expensive pumpingsystems. Typically, small positive displacement pumps (such aspiezoelectric pumps) are preferred, as these pumps are capable ofproviding a constant applied hydrodynamic pressure to the microinjector.Moving parts are also minimized or eliminated, as the atomization isaccomplished wholly or primarily through the applied voltages.

[0059] The atomizer of the invention is capable of very rapid andprecise control as droplet formation is dependent primarily on theapplied voltages rather than on changes in the operation of moving parts(i.e., no inertia associated with mechanical components or moving partsis present). This allows the atomizer to respond in real-time to changesin operating conditions in applications such as combustion engines.

[0060] For many applications, the atomizer contains multiplemicroinjectors, so as to form multiple droplet streams. Multiplemicroinjectors can be arranged in any geometrical relationship that issuitable for a particular application. An example of such an embodimentis shown in FIG. 2. In FIG. 2, atomizer array 21 includes base 22. Base22 is ring shaped, with central opening 26. Base 22 defines an enclosedinternal liquid reservoir. A plurality of microinjectors 23 as describedabove is provided on top surface 27 of base 22. Each such microinjector23 is in liquid communication with the enclosed reservoir, as is inlet24. The atomizer also includes a means for connecting the pin emittersof the microinjectors to a voltage source (not shown). In thisembodiment, microinjectors 23 are arranged in a circular pattern.However, the microinjectors can be arranged in any two or eventhree-dimensional array, as is suitable for a particular application.

[0061] When the atomizer has multiple microinjectors, it is possible tocontrol different microinjectors individually. In preferred aspects ofthe invention, the atomizer will include at least two sets ofmicroinjectors, each of which sets is operable independently of theother. The number of independently operable sets may be as few as two,but each set may include as few as one microinjector, in which case thenumber of independently operable sets will equal the number ofmicroinjectors. Any intermediate number of independently operable setsmay exist, and any number of microinjectors may be included in any set.Independent operation of the microinjectors is accomplished byseparately controlling the electrical field induced gradients for eachset of the microinjectors, i.e., by controlling applied voltage and/orcurrents independently for each set of microinjectors. Independentvoltage control is straightforwardly achieved through the appropriatedesign of circuitry, such as providing independent wiring and controlsystems for each set of microinjectors. Individualized microinjectorcontrol enables one to produce droplets of different sizes from each setof microinjectors, easily change the size of droplets made by each setof microinjectors, and to easily vary the rate at which droplets areproduced by each set of microinjectors. It further allows one to producevarious spray patterns using the atomizer, by selecting the geometricarrangement of the microinjector sets and/or by controlling the outputof each set of microinjectors. Multi-mode operation, in which differentmicroinjectors produce droplets at different rates or of differentsizes, can also be achieved without changing driving pressurerequirements between the different sets of microinjectors. Reduced flowrates can be achieved by operating only a portion of the sets ofmicroinjectors. This allows for simple linear scaling of mass flowrates, as mass flow rate is a function of the number of activemicroinjectors in operation (assuming the microinjectors are alldesigned and operated in the same manner).

[0062] The atomizer of the invention is particularly suitable forproducing fluid droplets of from about 1 to about 150, more particularlyfrom about 5 to about 50, especially from about 5 to about 30 μm indiameter. It is useful in a wide range of applications in which (1) fineliquid droplets are required to be produced, especially when thedroplets are desired to be of a uniform, controllable size, or (2) verysmall but controlled quantities of fluids are dispensed. An example ofthe first type of application is a carburetion system for internalcombustion engines. An example of the second type of application is thepreparation of samples for matrix assisted laser desorption ionization(MALDI) mass spectrometer analyses.

[0063] In internal combustion engine applications, performance can beenhanced if very fine (order of 5-30 μm) liquid fuel droplets (orliquid-solid fuel mixtures) of uniform size are produced and mixed withair (or oxygen or other liquid, vapor or solid oxidizers in the form ofa stream such as an air breathing engine or spray droplets for rocketengines operating outside the atmosphere) for injection into the enginecombustion chamber(s). This can be achieved by incorporating theatomizer of the invention into a carburetion system which (1) uses theatomizer to produce fuel droplets which (2) are then mixed with air atappropriate ratios and (3) provides the fuel/air mixture to thecombustion chambers. The atomizer is therefore configured to inject fueldroplets into a mixing zone where the droplets are mixed with the air,vaporize, and are provided the combustion chamber(s). The fuel/airmixture may be pulled into the combustion chamber via vacuum or injectedinto the chamber through a fuel injection system. The atomizer isadaptable for use in spark ignition engines as well as compressionignition engines. However, the benefits of the atomizer are particularlyseen in compression ignition engines, where fine particle droplets ofcontrollable size are produced using very low operating pressures, andin jet engines, where it is no longer necessary to depend on airturbulence to atomize the fuel. Suitable fuels include gasoline, dieselfuel, kerosene, various jet fuels, and the like.

[0064] The annular array shown in FIG. 2 is adaptable for use in such acarburetion system. Dispersed fuel droplets emerging from microinjectors23 are mixed with air which flows through central opening 26 in thedirection indicated by arrow 25. An advantage of this geometry is thatthe fuel droplets are sprayed into the shear layer where high turbulenceintensity will provide high mass transfer rates. The resulting mixturecan then be transferred to a combustion chamber for ignition. As shownin FIG. 2, the direction of droplet injection is roughly parallel to thedirection of airflow. If desired, the droplets can be injected into theairflow at some angle (including injecting the droplets into centralopening 26, perpendicular to the direction of the flow of the air).Similarly, additional air may flow, again in the general directionindicated by arrow 25, outside of the atomizer to further improvemixing.

[0065] Atomizers used in combustion engine applications preferablyinclude a plurality of microinjectors, in two or more independentlyoperable sets as described before. Independent operation of themicroinjectors enables precise and rapid control of overall flow rates(as total flow depends on the number of microinjectors in operation),fuel/air ratios (for the same reason), fuel droplet particle sizedistribution (if different sets of microinjectors produce different sizedroplets due to geometric design, or via variations in applied voltages)and droplet spray patterns.

[0066] This ability to control the operation of the atomizer permits itsoperation to be optimized on a real-time basis to adjust for changes inengine operating conditions or power requirements. The atomizer ispreferably computer-controlled in carburetion applications, the computermanipulating the voltage supplied to one or more sets of microinjectorsaccording to an algorithm that relates controls and/or informationregarding engine or other conditions to the operation of the varioussets of microinjectors. If preferred embodiments, the computer inaddition receives information regarding at least one engine or othercondition (such as operating temperature, oxygen availability, operatingspeeds, etc.) and adjusts the operations of one or more sets ofmicroinjectors in response to that information.

[0067] The invention also provides a method by which small volumes offluids can be atomized effectively. This characteristic makes theatomizer of the invention suitable in applications where small volumesof finely dispersed droplets are desired. Injection rates of less than 1μL/minute, especially from about 1-100 μL/minute are attainable, therebyproviding for controlled dispensing of very small quantities ofmaterials. If desired, higher mass flow rates can be obtained bychanging spray modes, increasing voltages, applying a pulsed voltage orincreasing the hydrodynamic pressure. It is an advantage of thisinvention that in many cases, a wide range of mass flow rates can beachieved using a particular microinjector and a particular fluid, byvarying one or more of these parameters.

[0068] An example of such an application is the preparation of samplesfor matrix-assisted laser desorption ionization (MALDI) massspectrometry. This technique is useful to characterize a number ofbiological materials such a proteins and genetic material. In MALDImethods, a sample is treated with a laser to ionize and volatilizesample molecules. The ionized and volatilized molecules are thenelectrostatically accelerated into a detector, with the flight timebeing measured. The flight time is then used to estimate the weight ofthe ion, and the weight is used as a tool for identifying the molecule.A very small amount of a sample is affixed to a sample slide, togetherwith a chromophore (which absorbs laser light well). Because the sampleis often an air-borne biological material, samples are often collectedby concentrating an air sample and directing the concentrated air sampleonto the sample slide. It is desirable to treat the air-borne biologicalmatter with various fluids in order to break open the cell wall ormembrane to expose the genetic material or proteins inside, add thechromophore to the sample, and/or apply a wetting or electrostatic agentwhich may simply help affix the material to the slide. A preferred wayof accomplishing this is to expose the biological matter to finedroplets of these fluids as the sample slide is prepared.

[0069] The atomizer of this invention is particularly well suited tocreating and applying treating fluids for MALDI sample preparation. Thematerial to be analyzed is dispersed in air or other gaseous carrier andallowed to flow through a spray chamber and onward to contact a sampleslide. The interior of the spray chamber includes a microinjector of theinvention, or, if more than one fluid is to be applied, a like number ofmicroinjectors. As the sample passes through the spray chamber, themicroinjector(s) are activated, each creating a spray cloud of dropletswhich contact the sample particles, thereby applying the desired fluidsto the sample. Typically, at least one of the sprayed fluids will be asolution of a chromophore such as trifluoroacetic acid. As eachmicroinjector can be operated individually, controlled, independentamounts of all fluids can be applied. Further, operating conditions foreach microinjector can be independently selected so as to optimizedroplet size and injection rates for each fluid.

[0070] The following examples are provided to illustrate the inventionbut not to limit the scope thereof All parts and percentages are byweight unless otherwise indicated.

Example 1

[0071] Isopropanol is fed to a 200 μm internal diameter stainless steelhypodermic needle with a pointed tip, with just enough appliedhydrodynamic pressure (less than 1 inch water) to maintain a steadystream of fluid to the needle tip. No droplets or mass flow out of theneedle is seen until a voltage is applied to the needle. A rectified,330 Hz, 3-4 kV voltage is supplied to the needle. Droplets (<100 μmdiameter) are formed in a single droplet mode. Injection velocity isestimated at 75 mm/s, with slowing due to air drag as the dropletstraveled. Power consumption can not be measured because of an extremelylow Lissajou current.

Example 2

[0072] Isopropanol is fed into a 100 μm internal diameter stainlesssteel needle with a sharp tip, under a pressure equal to approximately2″ water. An unpulsed DC voltage is applied to the needle. Approximately4000 volts DC are required to initiate atomization. At about 5000 volts,droplet formation assumes a spray mode with approximately 10 μm/minutemass flow rates. Current consumption at this voltage is about 40 μA.Droplets are very uniform in size and are approximately 30 μm indiameter. Further increasing the DC voltage decreases droplet size andincreases mass flow rates, droplet velocity, and dispersion angle.

[0073] The applied voltage is then changed to a 100 Hz, 10 kV rectifiedvoltage. A significantly higher mass flow rate, smaller dropletformation and smaller dispersion angle are generated, compared to whatis produced with a similar DC voltage. Further, the system can toleratehigher applied hydrodynamic pressures when a pulsed voltage mode ofoperation is used, as droplet formation is significantly faster.

Example 3

[0074] A mixture of ethanol and less than 0.1 weight percent bacterialspores is prepared. This mixture is atomized using a 620 μm (ID)stainless steel hypodermic needle with a square wave-driven (28 Hz), 20kV applied voltage and no applied hydrodynamic pressure. Fine dropletsin a spray mode are formed.

[0075] Similar results are obtained using a 220 μm (ID) needle, or whena 20 kV DC current is applied.

[0076] A mixture of water and less than 0.1 weight percent bacterialspores is prepared and atomized using the same 620 μm (ID) stainlesssteel hypodermic needle with a square wave-driven (28 Hz), 20 kV appliedvoltage and no applied hydrodynamic pressure. A bimodal spraydistribution is observed. The spray assumes a generally conical pattern,with the bacterial spores concentrated in the region near the axis ofthe cone. Similar results are seen using a 220 μm needle or a 20 kV DCvoltage.

Example 4

[0077] A mixture of 70 weight percent acetonitrile and 30% water (with0.1% trifluoroacetic acid) is atomized using a 100 μm (ID) stainlesssteel hypodermic needle with a 5 kV applied DC voltage and a smallapplied hydrodynamic pressure. A bimodal spray distribution is observed.The spray assumes a generally conical pattern, with the acetonitrileconcentrated in the region near the axis of the cone and the waterconcentrated near the periphery of the cone.

[0078] Similar results are seen when a mixture of fluorocene andisopropanol is atomized under similar conditions.

Example 5

[0079] A MALDI sample preparation apparatus is prepared with threeindependently controlled microinjectors that are oriented to sprayatomized liquids into a sample preparation zone. The injectors areoriented such that each sprays into the same region of the samplepreparation zone. The sample preparation zone is a channel,perpendicular to the orientation of the microinjectors. A concentratedgas stream containing the sample to be analyzed (such as bacteria sporesor other biological materials) is passed through the sample preparationzone, contacted with the sprayed fluids, and then directed onto a sampleslide for MALDI analysis. Each of the microinjectors is a 100 μm IDstainless steel needle connected to a square-wave driven, 20 kV, 5mA(peak-to-peak power) source. Each microinjector is supplied with processfluids from a separate fluid reservoir. Each reservoir is pressurized toabout 2″ water pressure. This hydrodynamic pressure provides a constantflow of fluids to the microinjectors.

[0080] The first microinjector is fed with isopropanol. The second isfed with a mixture of 70% acetonitrile, 30% water and 0.1%trifluoroacetic acid. The third is fed sequentially with various processfluids, including water, water/glycerine, acetic acid, formic acid andethanol.

[0081] The various microinjectors are first operated individually toassess the spray patterns that are produced. Ethanol, isopropanol,acetic acid and formic acid all form finely dispersed, uniformly sizeddroplets under these conditions. The acetonitrile/water/trifluoroaceticacid mixture forms a bimodal spray, with the acetonitrile dropletsconcentrated near the center of the spray and the water concentratednear the boundaries of the spray. The water/glycerine mixture forms asimilar spray pattern.

[0082] Water alone forms a bimodal spray, with a subset of largerdroplets being formed and dispersed at a wider dispersion angle thananother group of finer droplets. This bimodal distribution may be due toimpurities in the water being separated to a certain extent from thewater molecules. This creates relatively purified droplets that are lesshighly charged and form a fine mist, and droplets that are richer inimpurities (believed to include ionic species) which are more highlycharged and form larger, more widely dispersed droplets.

What is claimed is:
 1. An atomizer for a liquid comprising A) at leastone microinjector including (1) an orifice through which the liquid isbrought in contact with a pin emitter and (2) a conductive pin emitterextending outwardly from said orifice, the pin emitter having a radiusof curvature in at least one location external to said orifice of nogreater than 500 μm; B) means for introducing the liquid to be atomizedthrough the orifice and to the pin emitter, and C) means for connectingsaid pin emitter to a voltage source.
 2. The atomizer of claim 1,wherein said means for introducing the liquid to be atomized is adaptedto provide the liquid to the pin emitter under a hydrodynamic pressureof zero to 5 inches of water.
 3. The atomizer of claim 2 furthercomprising a voltage source that is adapted to supply a voltage of 100Vto 25kV to the pin emitter.
 4. The atomizer of claim 3 wherein thevoltage source is a variable voltage source adapted to supply a DCvoltage that can be varied in the range from 3 to 15kV.
 5. The atomizerof claim 3 wherein the voltage source is adapted to provide a pulsedvoltage to the pin emitter.
 6. The atomizer of claim 5 wherein thevoltage source is adapted to provide a voltage to the pin emitter thatis pulsed at a frequency of from 50-1000Hz and a peak-to-peak voltagefrom about 1-25kV.
 7. The atomizer of claim 1, wherein the pin emitteris a tip of a hollow needle and the orifice is the bore of the needle.8. The atomizer of claim 7, wherein the hollow needle has an insidediameter of 5 to 400 μm.
 9. The atomizer of claim 8, wherein the hollowneedle has a sharpened tip and the pin emitter is the sharpened tip ofthe needle.
 10. The atomizer of claim 1, which includes a plurality ofsaid microinjectors.
 11. The atomizer of claim 10, wherein the pluralityof said microinjectors include a first set of at least one microinjectorand a second set of at least one other microinjector, and said first setis operable independently of said second set.
 12. A method of producingliquid droplets comprising I) introducing a liquid into an atomizercomprising A) at least one microinjector including (1) an orificethrough which the liquid is brought in contact with a pin emitter and(2) a conductive pin emitter extending outwardly from said orifice, thepin emitter having a radius of curvature in at least one locationexternal to said orifice of no greater than 500 μm; B) means forintroducing the liquid to be atomized through the orifice and to the pinemitter, and C) means for connecting said pin emitter to a voltagesource; II) bringing the liquid into contact with the pin emitter, andIII) applying sufficient voltage to the pin emitter such that the liquidis emitted from the pin emitter as a plurality of droplets.
 13. Themethod of claim 12 wherein said liquid is brought into contact with thepin emitter under an applied hydrodynamic pressure of from 0 to 5 inchesof water.
 14. The method of claim 13 wherein the applied voltage is a DCvoltage of from 100V to 25kV.
 15. The method of claim 13 wherein theapplied voltage is pulsed.
 16. The method of claim 15 wherein thevoltage is pulsed at a frequency of from 50-1000Hz and a peak-to-peakvoltage from about 1-25kV.
 17. The method of claim 13 wherein the pinemitter is a tip of a hollow needle and the orifice is the bore of theneedle.
 18. The method of claim 17, wherein the hollow needle has aninside diameter of 5 to 400 μm.
 19. The method of claim 18, wherein thehollow needle has a sharpened tip and the pin emitter is the sharpenedtip of the needle.
 20. The method of claim 19, wherein the atomizercontains a plurality of said microinjectors.
 21. The method of claim 20,wherein the plurality of said microinjectors include a first set of atleast one microinjector and a second set of at least one othermicroinjector, and said first set is operable independently of saidsecond set.
 22. The method of claim 12 which produces liquid droplets offrom about 1 to 150 μm in diameter.
 23. The method of claim 12 whereinthe liquid is a mixture of two or more materials.
 24. The method ofclaim 23, wherein at least two materials in the mixture have differentdielectric constants, and the droplets formed disperse into a regionthat is enriched in one material and a second region that is enriched inanother material.
 25. The method of claim 24, wherein droplets from aregion that is enriched in one material are separated from droplets fromanother region that is enriched in another material, and collected. 26.A carburetion system for an internal combustion engine, comprising I) anoutlet for a mixture of atomized fuel droplets and air; II) an air inletwhich is in fluid communication with said outlet such that duringoperation air passes through said inlet, is mixed with fuel droplets andpasses through the outlet; III) an atomizer that is in fluidcommunication with said outlet and which emits a plurality of fueldroplets into a stream of air that passes from the air inlet to theoutlet, wherein said atomizer includes A) at least one microinjectorincluding (1) an orifice through which the fuel is brought in contactwith a pin emitter and (2) a conductive pin emitter extending outwardlyfrom said orifice, the pin emitter having a radius of curvature in atleast one location external to said orifice of no greater than 500 μm;B) means for introducing the fuel through the orifice and to the pinemitter, and C) means for connecting said pin emitter to a voltagesource.
 27. The carburetion system of claim 26 in which the atomizercomprises a plurality of said microinjectors.
 28. The carburetion systemof claim 27 wherein the plurality of said microinjectors include a firstset of at least one microinjector and a second set of at least one othermicroinjector, and said first set is operable independently of saidsecond set.
 29. The carburetion system of claim 27 wherein themicroinjectors are oriented to spray the fuel droplets into a highlyturbulent shear layer.
 30. The carburetion system of claim 28, furthercomprising a computer which controls the operation of themicroinjectors.
 31. The carburetion system of claim 28 wherein thecomputer is adapted to receive information regarding at least one engineor other condition and adjust the operation of one or more sets ofmicroinjectors in response to that information.